In parallel, an increase in the incidence of various adult diseases and central nervous system diseases, and an increasing aged population has posed a major social problem. How to treat patients with dementia, a central nervous system disease, in particular, is a quite an urgent problem to resolve. There have been no effective means of treating demented patients, especially those with senile dementia of Alzheimer's type or Alzheimer's disease, and prevention of the progression of dementia.
JP-A-3 81218 (U.S. Pat. No. 5,059,627) and JP-A-7 61923 (EP-A629400) disclose that idebenone is effective as a therapeutic agent for senile dementia of Alzheimer's type. JP-A-3 81218 discloses that the dosage of idebenone for adult humans is 0.1 mg to 500 mg per day. JP-A-7 61923 discloses a high dosage of idebenone of not less than 150 mg per day per adult.
In addition, Arch. Gerontol. Geriatr., Vol. 15, pp. 249-260 (1992) describes that idebenone proved to be effective in patients with senile dementia of Alzheimer's type when administered at 90 mg per day per adult.
However, the therapeutic methods described in these publications are characterized in that idebenone is administered for up to 6 consecutive months only and no more. There is no report regarding the potential of idebenone administration for safe prevention of dementia symptoms progression for an extended period of time.
In general, when administered at high doses, or for an extended period of time even at low doses, medicines often have adverse effects etc., which limit the desired patient treatment and hamper the obtainment of a satisfactory therapeutic effect. With regard to methods for preventing or treating dementia, especially senile dementia of Alzheimer's type and Alzheimer's disease, and prevention of the progression of dementia, no effective pharmaceuticals have been reported, except tacrine, a symptomatic drug. A long-term administration of tacrine is substantially impossible due to adverse effects such as hepatic toxicity. In fact, 74% of the patients receiving 30-week administration of tacrine withdrew from the treatment.
Against this background, the present inventors have found that excellent therapeutic effects or symptom progression-suppressing effects can unexpectedly be obtained very safely with minimum onset of adverse effects by administering idebenone for an extended period of time, especially for a period exceeding 6 months, as a method of preventing or treating and prevention of the progression of dementia, especially senile dementia of Alzheimer's type and Alzheimer's disease.